1. FIELD OF THE INVENTION
The present invention relates to a method of performing beam compression of an antenna pattern, which can provide better performance in the beam compression process of an antenna pattern of a radar including a monopulse power feed system.
2. DESCRIPTION OF THE RELATED ART
In general, a beam width is one of indices representing the performance of an antenna pattern of a receiving antenna or of other types of antennas. A narrower beam width in an antenna pattern will yield a better performance. However, there is an inverse relationship between a beam width and the size (length) of an antenna. Therefore, if the beam width is reduced, then the size of the antenna will be increased. Conversely, if the dimension of the antenna is reduced, then the beam width will be broadened.
For example, in an antenna for a radar system, if it is desired to double the ability or the resolution to discriminate objects, it is required to halve the beam width and thus it is required to double the size of the antenna. The doubling of the size leads to not only a larger occupied region but also various disadvantages such as an increase in the weight of the antenna and in the size of a structure for supporting the antenna. Conversely, if the size of an antenna is halved, then the beam width will be doubled and the discrimination ability will be degraded by a factor of two.
It is well known that there is such a conflicting relationship between a beam width and the size of an antenna. In most cases, an actual antenna has a limitation in the region it can occupy. Therefore, under these limited conditions, a certain degree of compromise associated with the beam width has to be made.
One known beam compression technique which alleviates the above-described problems is to employ two similar antennas with a monopulse power feed system so as to reduce the beam width by subtracting the difference signal pattern between the two antennas from the sum signal pattern of the two antennas. FIG. 1 is a schematic diagram illustrating a radar system which can perform the beam compression in such a manner as described above. In this figure, reference numerals 101,101 designate a pair of antennas with a monopulse power feed system, wherein each antenna has a length of a, and the center-to-center distance between the antennas is d. Reference numeral 102 designates a power divider and reference numeral 103 designates a transmission circuit. The transmission circuit 103 generates the transmission power, which is fed in the same phase to the antennas 101 via the power divider 102. Reference numeral 104 designates a hybrid circuit which produces a sum signal .SIGMA. and a difference signal .DELTA. from receiving signals of the two antennas 101,101. Reference numeral 105 designates a detector for detecting the sum signal .SIGMA. and the difference signal .DELTA., and reference numeral 106 designates a differential amplifier which provides an antenna output signal by subtracting the difference signal .DELTA. from the sum signal .SIGMA..
In the radar system having the configuration described above, the differential amplifier 106 provides the antenna output signal produced by subtracting the difference signal .DELTA. from the sum signal .SIGMA.. One aspect of the antenna output signal is shown in FIG. 2 in a general representing manner of a power pattern associated with beam compression. That is, in FIG. 2, the broken line represents the sum signal .SIGMA. of the receiving signals of the two antennas, and the alternate long and short lashed line represents the difference signal .DELTA.. The solid line represents the output signal (.SIGMA.-.DELTA.) which is the difference between these two signals. As can be seen, the output signal shows a synthetic directional characteristic having a compressed beam width.
A radar system which performs beam compression according to a conventional technique as described above can provide a certain degree of beam compression. However, if there are a large number of scattering objects in a direction of radio wave radiation, the difference signal .DELTA. will become small compared to the sum signal .SIGMA.. As a result, the final output signal value obtained by subtracting the difference signal .DELTA. from the sum signal .SIGMA. will have only a slight difference from the value of the sum signal .SIGMA.. This means that there is a problem in that efficient effects of beam compression cannot be achieved.